The Rotary Drill Division of Ingersoll-Rand Company markets vehicle-carried blasthole drills used in the mining industry to drill holes into which blasting charges are inserted and set off to fracture rock formations. Typically, an operator relies on past experience to manually control the axial force applied to the drill bit and the rotational speed of the drill bit, the control being exercised by operation of manual controls provided in the operator's cab of the vehicle. The operator must continuously monitor the drilling operation, varying the rotational speed of the bit and the axial force applied to the bit as the bit moves through rock, less dense material and voids. The constant supervision results in operator fatigue. Furthermore, if the operator sets the rotation speed too high, vibration may be induced which could damage the drill. Also, in very soft material, applying too high an axial force could create such a volume of rock chips that the forced air removal system could not handle them and that could lead to plugging the air jets in the bit and result in interruption of the drilling process to clear the jets. In addition, an excessive axial force applied to the bit adversely affects the useful life of the bit.
Extensive field testing of rotary drills having tricone bits has shown that for a given type of rock the penetration of a bit, per revolution of the bit, is a unique function of the axially directed force applied to the bit and, for efficient utilization of the bit, the penetration of the bit per revolution (P/R) should not grossly exceed the height of the cutting elements of the bit. Tests further show that for a given axial force (FB), the absolute rate of advance of the bit into the formation is a unique function of the angular speed (N) of the bit. The limiting factor on how fast a bit can be rotated is the onset of drilling vibration. Finally, it has been observed that if the pressure in the forced air removal system rises above a normal operational value for a given drilling system, such rise is an indication that the advance of the bit per revolution of the bit is too high and plugging of the nozzles in the bit could occur.
Jasinski U.S. Pat. No. 4,793,421 discloses a control system wherein the axial force applied to a bit and the rotational speed of the bit are controlled at the maximum values possible without causing components and/or subsystem overloading to occur. This system does not control P/R to any reference value.
Zhulovsky et al. U.S. Pat. No. 4,354,233 discloses a control system for controlling P/R and the product (FB)(N), these values being referred to as Z and F/Z, respectively, in the patent. Z and F/Z are controlled so as to be as close as possible to values Z.sub.0 and (F/Z).sub.0 that are continuously calculated by a microprocessor. Z.sub.0 and (F/Z).sub.0 are presented as the optimum values for the drill to operate at for any given type of rock, based on various criteria built into the logic of the microprocessor. As the drill bit passes from one type of rock to another, the microprocessor calculates the appropriate values of Z.sub.0 and (F/Z).sub.0 for Z and F/Z to be compared against. A two-step control sequence is employed. If something causes Z to not equal Z.sub.0, a signal is sent to a rotation frequency regulator that in turn causes the bit rotation speed to change. This in turn causes F/Z to not equal (F/Z).sub.0 so a signal is sent to an axial load regulator to change the axial force applied to the bit.
From extensive field test data, it can be shown that P/R (Z in the reference) is in no way affected by a change in N alone. Therefore, the first step in Zhulovsky produces no direct result in terms of a change in P/R. It is only because of the second step (changing the axial force) that any change in P/R occurs.
Because F/Z in Zhulovsky is actually the product of FB and N, any change in N will produce a reciprocal change in FB for the product to remain constant. On the other hand, it is an operational advantage to have FB and N independently controllable.